1. Field of the Invention
This invention relates to optical rotation sensors; and, particularly, this invention relates to a split gain multi-mode ring laser gyroscope, having an active medium gain which is radio frequency excited, where a common helical resonator coil is used to perform gain medium excitation and mode suppression functions.
2. Description of the Related Art
Ring laser gyroscopes are an alternative form of rotation sensors which do not require the use of a spinning mass characteristic of a mechanical gyroscope. A ring laser gyroscope employs a Sagnac effect to detect rotation optically, as an alternative to the inertial principles upon which a mechanical gyroscope operates.
The earlies developed ring laser gyroscopes have two independent counter-rotating light beams or other electromagnetic propogation which travel within an optical ring cavity. These two light beams propogate in a closed loop with transit times that differ ideally in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. Planar gyros are usually driven by a DC discharge power source where an active gas medium within the cavity is excited between a cathode and at least two anodes. In certain ranges of current operation, instabilities in the current and voltage discharges arise. Planar ring laser gyroscopes must be substantially symmetrical to counteract the potential for false reading due to the electrophoretic process known as Langmuir flow.
An additional and more serious cause of inaccuracies in rotational sensing of a two-mode planar ring laser gyroscope is the phenomenon known as frequency lock or mode locking. Mode locking is a major difficulty at low rotation rates where the ring laser gyroscope produces a false indication that the device is not rotating. If the rotation rate of a ring laser gyroscope starts at a value above that of where lock-in occurs, and is then decreased, the frequency difference between the beams disappears at a certain input rotation. This input rotation rate is called the lock-in threshold. The range of rotation rates over which lock-in occurs is generally called the dead band of the ring laser gyroscope. Lock-in arises from the coupling of light between the beams. Today, the only means of overcoming the lock-in effect of the counter-propagating modes of light within a two mode gyroscope is to mechanically dither the mirrors or body of the gyroscope. A more detailed explanation of the problems associated with a planar two mode gyroscope are described in Laser Applications, edited by Monte Ross, Pages 133-200 (Academic Press, 1971).
Since one of the primary benefits of a ring laser gyroscope is that it overcame the need for mechanical or moving parts, a body dithered planar two mode gyroscope does not truly meet this goal. In an effort to achieve a fully optical ring laser gyroscope, the non-planar multi-mode ring laser gyroscope was developed to overcome the effects of mode locking without the need to dither. The term (multioscillator) refers to four modes of electromagnetic energy that propagates simultaneously in the cavity as opposed to the usual pair counter-propagating linearly polarized modes that exist in the conventional two mode gyroscope. A detailed discussion of the operation of the multi-oscillator laser gyroscope is presented in the article Chow, et. al., at pages 918-936, IEEE Journal of Quantum Electronics, Vol. QE-16, No. 9, September 1980.
Briefly, the basic multi-oscillator ring laser gyroscope operates with left circularly polarized (LCP) and right circularly polarized (RCP) light beams and uses a Faraday effect glass device within the cavity or magnetic field on the gain plasma to provide a phase shift between the counter propagating waves to prevent mode locking. Reflections and backscatter from the intra-cavity element and instabilities of the magnetic field associated therewith cause difficulties that need to be overcome in order to build a fully optical navigational grade multi-oscillator ring laser gyroscope.
An important attempt to overcome the problems presented by the multioscillator ring laser gyroscope is the split gain multimode ring laser gyroscope as disclosed and claimed in U.S. patent application Ser. No. 07/115,018, filed Oct. 28, 1987 (placed under Secrecy Order on May 17, 1988). The split gain multimode ring laser gyroscope is directed to an non-planar multimode ring laser gyroscope, having no intra-cavity element. The split gain gyroscope includes the step of adjusting an axially applied magnetic field to a magnitude that produces a splitting between the gain curve for anti-clockwise left circularly polarized light (La) and clockwise right circularly polarized light (Rc) and the gain curve for clockwise left circularly polarized light (Lc) and anti-clockwise right polarized light (Ra) that is substantially equal in frequency to a multiple of the free spectral range of the cavity. By providing an axially directed magnetic field to the gain medium, the lasing action of selected modes of the cavity is suppressed by means of frequency shifting the gain curve centers, preventing frequency locking. While originally designed for use with a DC discharge mechanism (for exciting the active gain medium), an RF excited gain medium would provide a most desirable design.
As taught in U.S. patent application Ser. No. 07/115,018 (assigned to common assignee of this application), radio frequency discharges may be used to excite the gain medium of a split gain gyroscope. The split gain gyroscope also requires that where a permanent magnet is used to provide an axial magnetic field, a DC helical coil is also needed in order to fine tune the magnetic field to properly split the gain curves within this multimode ring laser gyroscope. In particular, FIG. 15 of U.S. application Ser. No. 07/115,018 discloses a configuration for achieving radio frequency excitation of a split gain multimode ring laser gyroscope.
The design described in FIG. 15 of the Ser. No. 07/115,018 U.S. Patent Application is reproduced as FIG. 1A of this application. With reference to the prior art figure 1A in this application, it will be noted that a gyro frame 12, along one of its frame legs supports a gain medium excitation resonator helical coil 20, which is wrapped around the frame leg of the gyro frame 12. Surrounding the helical 20 is resonator shield 18, which may be a cylindrical copper tube open at each end only enough to accommodate positioning along the frame leg of the gyroscope frame 12. Immediately surrounding and enclosing a resonator shield 18 are DC field windings 16 used to fine tune the axial magnetic field, applied by the permanent magnetic 14 to the active gain medium 22. It can be seen that this design (FIG. 1A) is rather complicated and requires two separate coils, one to carry a DC signal (16) and another coil to carry the AC excitation signal (20).
With reference to FIG. 1B, it will also be noted that the axial magnetic field produced has low gradient characteristics (the flat portion of curve 24 of FIG. 1B) over an axial distance that is less than the axial length of the gain medium 22. It has recently been discovered that low gradient throughout the gain medium 22 is desirable in order to reduce thermal bias effects. Also, the split gain effect is enhanced when the magnetic field is provided uniformly across the active gain medium 22 region. The design shown in FIG. 1A does not easily accomplish the task of providing a low gradient magnetic field through the gain medium 22. Since the split gain multimode ring laser gyroscope operates best when the active medium is contained within the permanent magnet 14, the radio frequency excitation mechanism shown and described in FIG. 1A (and U.S. patent application Ser. No. 07/115,018) does not present a design showing a magnetic field which is truly uniform throughout the entire extent of the gain medium 22. Also, this prior art design (FIG. 1A) is too cumbersome to easily manufacture.